Corrosion resistant pipelines for the submarine or otherwise underwater transportation or conveying of corrosive fluids such as gas or crude oil can be provided by pipes having an internal metallic liner. A double-walled or bi-metallic pipe is generally composed of two metallic layers. The outer layer is to provide resistance against buckling on the reel or sea bottom and provides general strength to the pipe so as to resist to hydrostatic and internal pressure, whilst the internal layer protects the outer layer from damage (corrosion) due to the chemical composition of the fluid being conveyed, and may also provide strength (e.g. for pressure containment). The inner layer is sometimes also termed a “liner”. As the main purpose is to protect the outer layer from corrosion, a corrosion resistant alloy (CRA) is commonly chosen as the liner.
One form of bi-metallic pipe can be termed a mechanically lined pipe (MLP), where a liner is fixed to the outer layer (such as carbon steel) without metallurgical bonding. An economical method of forming a lined pipe uses hydraulic and/or mechanical expansion, where the liner is inserted into the outer layer, and then both parts are expanded. During the expansion, the inner pipe undergoes a plastic deformation while the outer layer undergoes either elastic or plastic deformation, depending on the manufacturing process.
One example of this comprises inserting an alloy 316L liner inside a carbon steel host pipe, and expanding the liner radially so that it comes into contact with the host pipe. Then the host pipe outer diameter will also expand together with the liner to a pre-determined strain level such that, following relaxation of the internal pressure, an interference contact stress between the liner and the host pipe remains.
The liner at the ends of each pipe section of the MLP is sealed to avoid water and/or moisture ingress between the outer pipe and the liner. This may be achieved by a ‘clad overlay weld’ at the end of each first pipe length or ‘pipe joint’. According to common practice, clad overlay welds are 50 mm long to enable inspection and, if needed, repair of the pipeline girth welds conjoining the pipe joints.
There are two common methods of laying underwater or submarine pipelines. The so-called ‘stove piping method’ involves joining pipe stalks together on a pipe-laying vessel by welding each one as the laying progresses. In the so-called ‘reeled laying method’, the pipeline is assembled onshore and spooled onto a large reel, sometimes also termed a storage reel or drum. Once offshore, the pipeline is spooled off from the reel, aligned, straightened and finally laid on the seabed. In this method, no welding is required during the offshore operation, saving time for the vessel operation.
The reeled laying method is faster and more economical than the stove piping method, such that it is preferred where possible. However, the reeling process obviously generates significant multiple bending strains in the pipeline, which would cause a conventional 2.5-3.0 mm liner in a conventional lined pipe to wrinkle, and it is currently considered that wrinkles are detrimental to an MLP. Thus, all current methods developed to load an MLP onto a reel have been based on the idea that the formation of any wrinkles should be avoided at all costs during the spooling processes.
WO 2008/072970 A1 discloses a method for laying a pipeline having an inner corrosion proof metallic liner that is held inside an outer pipe material by interference stresses. In its method, a section of the pipeline is reeled onto a pipe laying drum, whilst an overpressure is maintained within the section by means of a pressurised fluid. When the pipeline is motionless, the overpressure is relieved, and a further pipeline section is joined to the first section. A new overpressure is then applied within the sections, and the further section is reeled onto the pipe laying drum. This requires the overpressure and pressure-relieving steps every increment when two pipe sections are joined. The pipe laying drum is described in WO 2008/072970 A1 as typically having installed “many” pre-fabricated sections, creating significant repetition of the overpressure and pressure-relieving steps required.
WO2011/048430A describes a method of reel-laying a mechanically lined pipe (MLP) comprising spooling the MLP onto a reel in the complete or substantial absence of internal pressure above ambient pressure in the MLP, where the MLP has a defined liner thickness, to thereby provide an unspooled MLP having wrinkles of less than 4 mm, which can then be removed by pressuring the pipe during pre-commissioning of the MLP.
However, because of inherent variations in diameter, wall thickness and yield strength between MLP joints, there is a possibility that during spooling, the bending moment required to force an MLP pipe length or pipe joint to bend onto the reel may be significantly higher than that required to bend an adjacent MLP pipe joint (i.e. the bending moment capacity of the first joint is significantly higher than that of the adjacent joint). In such an instance, a high compressive strain localises in the joint with a lower bending moment capacity (i.e. the ‘weaker joint’), generally near the girth weld joining the two pipe joints. As a result, the weaker joint may suffer from a high local ovality, and the liner may wrinkle at this location, which is clearly undesirable.
The difference in bending moment capacities between successive pipe joints is termed, in the art, “bending moment mismatch” or even just “mismatch”. Mismatch is described in more detail in the ASME paper no. OMAE 2011-49389, presented at the OMAE conference in the Netherlands in 2011. This document is incorporated herein by way of reference. The Paper Abstract confirms that natural variation of wall thicknesses and yield strengths determines the potential differences in bending stiffness. These mismatches cause a localised peak in strain and can drive gross deformation of the pipe, which may result in a buckle if not addressed at the engineering stage. The level of mismatch (MM) between two pipes is determined from:
  MM  =      2    ⁢                            M          p_s                -                  M          p_w                                      M          p_s                +                  M          p_w                    whereMp_s is the bending moment capacity of a strong pipe joint, andMp_w is the bending moment capacity of a weak pipe joint.Mp_s and Mp_w are determined from
      M          p_s      /      w        =                    σ                  y_s          /          w                    6        ⁢          (                        OD                      s            /            w                    3                -                  ID                      s            /            w                    3                    )      whereσy_s/w is the yield strength of a strong (or weak) pipeODs/w is the outer diameter of a strong (or weak) pipe, andIDs/w is the inner diameter of a strong (or weak) pipe.
To counteract the effect of mismatch, the thickness of the outer pipe and/or the thickness of the liner could be increased, but both of these options also significantly increase the overall cost of the MLP.